Implement shared packages (tasks 2.1-2.6)

- @trails-cool/types: Route, Activity, Waypoint, RouteVersion, RouteMetadata
- @trails-cool/gpx: GPX parser (XML→waypoints/tracks/elevation) and generator
  with round-trip test, haversine distance, elevation gain/loss computation
- @trails-cool/map: MapView (Leaflet + OSM/OpenTopoMap/CyclOSM layer switcher),
  RouteLayer (GeoJSON polyline rendering)
- @trails-cool/ui: Button (primary/secondary/ghost), Input (with label/error),
  Card components with Tailwind styling
- @trails-cool/i18n: react-i18next config with English + German translations,
  browser language detection, fallback to English

All 13 unit tests pass. Both apps build successfully.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/map/src/layers.ts

@@ -0,0 +1,29 @@
+export interface TileLayerConfig {
+  name: string;
+  url: string;
+  attribution: string;
+  maxZoom?: number;
+}
+
+export const baseLayers: TileLayerConfig[] = [
+  {
+    name: "OpenStreetMap",
+    url: "https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png",
+    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a>',
+    maxZoom: 19,
+  },
+  {
+    name: "OpenTopoMap",
+    url: "https://{s}.tile.opentopomap.org/{z}/{x}/{y}.png",
+    attribution:
+      '&copy; <a href="https://opentopomap.org">OpenTopoMap</a> (<a href="https://creativecommons.org/licenses/by-sa/3.0/">CC-BY-SA</a>)',
+    maxZoom: 17,
+  },
+  {
+    name: "CyclOSM",
+    url: "https://{s}.tile-cyclosm.openstreetmap.fr/cyclosm/{z}/{x}/{y}.png",
+    attribution:
+      '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> &copy; <a href="https://github.com/cyclosm/cyclosm-cartocss-style/releases">CyclOSM</a>',
+    maxZoom: 20,
+  },
+];